Urea and ammonium nitrate are today the two main sources of nitrogen to world agriculture. Mainly used as straight nitrogen fertilizer, they are also currently combined into liquid fertilizer (UAN solutions, (urea ammonium nitrate)).
Urea and ammonium nitrate are considered as incompatible to prepare in solid blends. It is referred to proceeding no 558 from the International Fertiliser Society in London: “Introduction to guidelines for the production and handling of blended fertilisers”, presented on Apr. 14, 2005.
This is due to the highly hygroscopic double salts they form, when in contact with each other (see table hereunder). Formation of such salts leads to an extra liquid phase, absorption of water from surrounding atmosphere and in extreme cases, mud formation of the whole blend.
Urea and ammonium nitrate products of good quality have a low water content. They comprise far below 1 weight % of free water, usually some 0.2 weight % free water or less. However, even such small water amounts are sufficient to allow reaction between urea and AN to start and further develop, leading to caking and/or mud. This reaction can occur even when the product cannot absorb any water from surrounding atmosphere when bagged or stored in closed box, simply from the water content of the products before blending. The higher the storage temperature, the more the blend would then dissolve in its own water content.
Table 1 shows the critical relative humidity at 30° C. for various products.
TABLE 1Urea-ammoniumAmmoniumnitrate doubleProduct:ureanitratesaltsCritical relative ~75%~60%Below 20%humidity at 30° C.
The critical relative humidity of a fertilizer is defined as the atmospheric humidity at which the solid product absorbs exponentially water from atmosphere.
These properties of high solubility/high hygroscopicity allow manufacturing of UAN solutions, which are popular liquid fertilizers presenting numerous advantages, a non-freezing fertilizer combining various sources of nitrogen (ammonium, nitrate, urea) at high nutrient concentration. Since their salt out temperature is well below 0° C., standard UAN grades are 28%, 30% and 32% nitrogen. In tropical countries, not subjected to freeze, even more concentrated solutions can be used. Table 2 shows the composition and crystallisation temperature for different UAN grades.
TABLE 2UAN grade:28% N30% N32% NWeight % AN40.142.243.3Weight % urea30.032.735.4Weight % water 29.925.120.3Crystallization−18−10−2temperature ° C.
The classically accepted incompatibility between ammonium nitrate and urea mixed in dry blends is basically due to the formation of double salts, making such dry blends impossible to produce. To get double salt formation, liquid phase is actually required so that the compound can react together.